1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus that transmits ultrasonic wave to a living organ and receives the echo from it, detects the degree of coincidence between two scanning data and acquires the least moving ultrasonic image when the organ moves.
2. Description of the Related Art
A known ultrasonic diagnostic apparatus transmits ultrasonic wave to a living tissue and then produces a tomographic image or video of the living tissue based on the echo signal returned from it. Now in widespread use in this field today is such an ultrasonic diagnostic apparatus that uses, as a transducer for transmitting ultrasonic wave to and receiving the echo signal from a target region, a mechanical scanning ultrasonic endoscope having an ultrasonic transducer on its end or an ultrasonic probe having no optical system such as image guide, which is inserted into a body cavity to diagnose the target region in an endoscopic manner.
Such an ultrasonic diagnostic apparatus having an ultrasonic endoscope allows its ultrasonic transducer to mechanically scan. In the course of diagnosing process, the living tissue may be agitated (for example, when the body of a subject moves), or an insertion portion of the endoscope may be shaken particularly when it is soft tissue. Thus, the region of the tissue with which the transducer is contact is also moved, and a resulting tomographic image is substantially distorted.
When a PPI (Plan Position Indicator) scanning, typically employed in many of radar installations, is performed, tomographic images occasionally suffer discontinuity in presentation due to movement of the region to be probed when the body or transducer is agitated. The resulting images are thus distorted and image quality is degraded.
To improve poor quality image, Japanese Patent Application No. 4-343839 has disclosed a monitoring apparatus in which display start position modifier means for ultrasonic image is provided so that the degradation of a tomographic image is prevented by an operator's own operation.
During diagnosing process using an ultrasonic diagnosing apparatus, an operator must use both hands to insert an ultrasonic endoscope or ultrasonic probe into the body cavity. The above disclosed method in which a degraded image is corrected by manipulating the display start position modifier means is neither easy nor practicable. Furthermore, in the above disclosure, if degradation of an image due to agitated body or transducer (ultrasonic transducer) affects a plurality of frames that started before stopping of scanning operation, correction action to the image will not work.
U.S. Pat. No. 4,846,188 has disclosed an ultrasonic diagnosing apparatus in which heart beat interval is divided by a number into a plurality of segments, and scanning sector is divided by the same number. An ultrasonic image is obtained in synchronism with the segments of the heart beat intervals. This technique, however, is not applicable to the case where image acquisition requires frame rate far faster than the heart rate.
The human heart beats about once a second, while the frame rate of the typical ultrasonic image ranges from 1/30 second to 1/10 second. The frame rate on the order of the human heart rate is only applicable to the special case where the imaging acquisition requirement is slow. Furthermore, the heart beat varies in response to a change in its surrounding situation. The above disclosed technique has thus difficulty with ordinary ultrasonic imaging application.